Stellar activity cycles: observing the dynamo?

Abstract

Abstract. The enormous complexity of the atmospheric structure observed on the Sun makes it very difficult to compare the Sun with "solar-type stars". Clearly, we need to identify parameters that can be observed on the Sun as well as on other stars which can be interpreted unambiguously. The most widely accepted dynamo signature is the presence of an activity cycle, well documented for the Sun and for main-sequence stars due to the Mount Wilson Ca II H&K project. Only recently have we detected spatial information, differential rotation and possibly meridional flows on other stars and thereby adding another constraint for its interpretation within a dynamo theory. Again, the picture is not complete yet, despite that there is just a single main ingredient that acts as the driving mechanism for activity in all atmospheric layers and the convective envelope of a solar-type star: the dynamo-related magnetic field. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action over long periods of time.

Stellar Chromospheric Activity

“…Section 3.5: Details of dynamo research and its numerous outstanding questions are outside the scope of this review; for a recent evaluation see Charbonneau (2005), and for summaries of Living Reviews in Solar Physics http://www.livingreviews.org/lrsp-2008-2 the long-term observational programs within the dynamo context, see Baliunas et al (1996) and Strassmeier (2005). Section 3.6: In several places in this review, I make note of the evanescent nature of the chromosphere that these recent observations clearly support.…”

Starspots: A Key to the Stellar Dynamo

“…Previous short reviews on starspots and their role in understanding the stellar dynamo were given by Hall (1991a), Lanza and Rodonò (1999a), Berdyugina (2004) and Strassmeier (2005). Also, starspots were thoroughly discussed at the international workshop "Surface Inhomogeneities on Late-Type Stars" (1990, Armagh), IAU Symposium 175 "Stellar Surface Structures" (1995, Vienna) and the First Potsdam Thinkshop " Sunspots and Starspots" (2002, Potsdam).…”

Starspots: signatures of stellar magnetic activity

“…Because the Sun does not show spots in excess of ±40 • (but see Tsuneta 2008 for the recent discovery of high-latitude kG magnetic fields), this discovery spurred significant interest in these types of stars, and in starspots in particular. Today, dark photospheric spots and bright chromospheric plages are still the main tracers for detecting stellar rotation periods using photometry and, if long time series are available, also for detecting differential rotation and magnetic-cycle periods (Donahue et al 1996, Rodonó et al 2001, Strassmeier 2005. Figure 1 is a recent data example and shows almost 20 consecutive years of automated V -band light curves of the spotted star HD 6286 (Strassmeier et al 2008a).…”